1. Field of the Invention
The present invention relates to a display device in which a thin film transistor is used at least in a pixel portion, and to a method for manufacturing the display device.
2. Description of the Related Art
In recent years, technology for fanning thin film transistors using thin semiconductor films (with thicknesses of from several nanometers to several hundreds of nanometers, approximately) formed over a substrate having an insulating surface has been attracting attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed.
As a switching element in an image display device, a thin film transistor including an amorphous semiconductor film, a thin film transistor including a polycrystalline semiconductor film, or the like is used. As a method for forming a polycrystalline semiconductor film, a technique is known in which a pulsed excimer laser beam is processed into a linear shape with an optical system, and an amorphous semiconductor film is scanned with the linear beam, thereby being crystallized.
As a switching element in an image display device, further, a thin film transistor including a microcrystalline semiconductor film is used (see Reference 1: Japanese Published Patent Application No. H4-242724; and Reference 2: Japanese Published Patent Application No. 2005-49832).
As a method for manufacturing a thin film transistor including a microcrystalline semiconductor film, a method is known in which after forming an amorphous silicon film over a gate insulating film, a metal film is formed thereover, and the metal film is irradiated with diode laser, whereby the amorphous silicon film is changed in quality to be a microcrystalline silicon film (for example, see Reference 3: Toshiaki Arai et al., “SID 07 DIGEST” 2007, pp. 1370-1373). According to this method, the metal film formed over the amorphous silicon film is formed to convert optical energy of the diode laser into thermal energy, and needs to be removed later in order to complete a thin film transistor. That is to say, in the above method, an amorphous semiconductor film is heated only with heat conduction from a metal film, thereby fanning a microcrystalline semiconductor film.